1. Field of the Invention
This invention relates generally to devices that are capable of delivering high-energy electrical pulses of very short duration.
2. Description of Related Art
Very large energy pulses (of the order of 10.sup.6 Joules) of short duration (of the order of milliseconds) are required for commercial applications such as welding and igniting fusion reactors and for defense applications such as electromagnetic rail guns, laser weapons, and active sonars. Many applications, particularly those involving space-based or mobile terrestrial power systems, require lightweight and/or low volume power sources.
There is also need for power sources which can supply electrical energy pulses of varying duration, over the range from microseconds to seconds. Moreover, it is desirable to vary the voltage of the power source over wide limits, e.g. from 50 to 1000 V or more, depending on the application.
A further requirement for some applications is the ability to deliver a rapid series of electrical pulses, for example, five pulses per second in the case of an electromagnetic hypervelocity launcher. Another desirable feature of a high-power pulse source is the ability to sustain nearly constant voltage for the duration of each high-current pulse.
Presently, high energy pulses of electrical power are typically provided by a large bank of capacitors or by a homopolar generator (HPG). Although capacitors can deliver pulses of relatively high power per unit mass (e.g. 100 kW/kg), their specific energy is typically only in the range of 0.1 to 2 kJ/kg. Thus, capacitors require a high mass (and consequent high volume) and are thus limited in their applicability. Moreover, the voltage of capacitors typically declines exponentially with time during discharge. This rapid decline significantly complicates the design of the equipment which uses the power pulses.
The HPG is a flywheel of conductive material rotating at high speed in a strong magnetic field which produces electric current. The HPG typically has specific energy of 2 to 10 kJ/kg with specific power of the order of 100 kW/kg. While the HPG is capable of delivering megaampere pulses, the shortest pulse length is of the order of 200 milliseconds and the HPG is limited to a maximum of several hundred volts. The HPG has other limitations which affect its use: it is an expensive machine having moving parts requiring repair or replacement; it is constructed of copper, iron, and other structural materials making it heavy and thus expensive for space-based applications; and the gyroscopic action of the HPG makes movement and guidance in a space-based system difficult.
Batteries, which have specific energies in the range of 100 to 750 kJ/kg, surpass HPGs and capacitors by two orders of magnitude or more in specific energy. The volumetric energy density of batteries shows similar gains over other energy storage technologies. Batteries can supply power pulses with durations ranging from the microsecond region (similar to capacitors) to the millisecond region (similar to HPGs) and beyond. Series-connected batteries can be built with any integer multiple of the single-cell voltage and thus are not limited to low-voltage operation as is an HPG, or high-voltage operation as are capacitors. A battery can supply identical, rapid, repetitive pulses without recharge.
Despite these advantages, battery power sources have not been widely used in high-power, pulsed modes of operation because the specific power of conventional batteries is far too low, typically around 0.1 to 1 kW/kg. Low specific power is the result of the use of electrochemical systems with slow electrode kinetics, electrolytes with low conductivity and battery designs with large cell and intercell resistances.
The limitations of conventional battery systems can be eliminated or significantly reduced by the use of a high-temperature, molten-salt secondary battery of bipolar construction. In particular, lithium alloy, metal sulfide (Li-MS) batteries with molten alkali halide electrolytes have been shown to be capable of meeting the current densities required of high-power pulse batteries.
Power in a battery of essentially ohmic character is given by: EQU P=(V.sub.o -IR.sub.i)I (1)
where the open circuit voltage V.sub.o and the effective internal battery resistance Ri are, in general, functions of the state of charge.
Differentiating the expression with respect to I at a particular state of charge gives EQU .differential.P/.differential.I=V.sub.o -2IR.sub.i ( 2)
At maximum power .differential.P/.differential.I=0, so that EQU V.sub.o/ 2R.sub.i =I. (3)
Equation (3) shows that for a battery whose polarization losses are minimal the current at maximum power varies directly as the open-circuit potential and inversely as the internal resistance. A well designed Li-MS battery will approach these conditions.
The open-circuit potential depends on the thermodynamics of the electrochemical cell reaction; it lies in the range of about 1 volt to 4 volts for all known practical battery systems. Battery current, and consequently battery power, is thus predominantly controlled by the internal resistance, which can vary by several orders of magnitude depending on the design and construction of the battery. Decreasing the internal resistance increases current and power.
The measured internal resistance of a battery can be considered as the sum of three components: the electronic resistance of the current collectors, the ionic resistance through the battery electrolyte, and other resistive elements related to the electrochemical kinetics of the cell reaction. These resistive components are essentially series in nature so each individual component must be as low as possible to provide minimal internal resistance.
Electronic resistance can be minimized by adopting a series-bipolar configuration. Most batteries use a flooded electrolyte or have other mechanical properties which complicate the use of a bipolar construction. Li-MS, on the other hand, is ideally suited for bipolar construction, and several experimental bipolar batteries have been assembled and tested by Gould Inc. (Rolling Meadows, Illinois) with good results.
Reducing the ionic resistance of the cell requires a high-conductivity electrolyte. Molten salts have the highest conductivity of any electrolyte. The conductivities of the lithium halide salts used in a Li-MS cell are five times higher than of the sulfuric acid electrolyte used in lead-acid batteries, which has the best conductivity of conventional batteries.
Because the rates of electrochemical processes increase exponentially with temperature, the high operating temperature of a Li-MS cell results in extremely fast electrode reaction kinetics. The apparent cell resistance associated with electrochemical activation polarization is thus negligible in comparison with the electrolyte resistance. However, concentration polarization effects may be observed depending on the cell design and composition and construction of the electrodes.
No other battery system has this combination of attributes to produce high power density. This can be seen in the recent development of molten-salt, primary, thermal batteries built for missile and nuclear weapon applications.
For example, C. S. Winchester describes a LAN/FeS.sub.2 thermal battery system in the Proceedings of the 30th Power Sources Symposium, Electrochemical Society, Inc., Jun. 7-10, 1982, pp. 23-27. The LAN anode is a mixture of lithium and an ultrafine metal powder; the electrolyte is a mixture of LiCl and KCl. Current densities achieved by the LAN/FeS.sub.2 cell at an operating temperature of 750.degree. C. ranged from 0.99 A/cm.sup.2 to 11.7 A/cm2 with peaks of 16.5 A/cm.sup.2 and 14.5 A/cm.sup.2. Specific power for the LAN/FeS.sub.2 battery was almost 2.7 kW/kg.
J. Q. Searcy and J. R. ArmiJo describe a thin cell Li(Si)/FeS.sub.2 thermal battery in the Proceedings of the 30th Power Sources Symposium, Electrochemical Society, Inc., Jun. 7-10, 1982, pp. 27-30. They developed a thin cell Li(Si)/FeS.sub.2 thermal battery to replace Ca/CaCrO.sub.4 in applications requiring small batteries. The electrolyte used was a LiCl-KCl mixture, and the cell achieved a specific power of nearly 3.4 kW/kg.
While the power performance of each of these batteries is better than those of conventional batteries, the specific power still is not near that required for applications such as advanced weapons. Moreover, these batteries are primary batteries and the lack of rechargeability severely limits their applicability.